To ensure insulating properties when submerged, although various cell cases of sealed structure have been proposed, it has been the issue to suppress capacity loss due to thermal degradation (reduction of cell life) of the cell accommodated inside. The radiation performance has been improved for the cell which becomes hot due to heat generated during power generation by providing a fin structure outside the housing (e.g., Japanese Patent Application No. JP 2004-304881 A), conducting heat to a metal plate of the housing (e.g., Japanese Application No. H10-333782), or making the outer surface of the housing to be of high emissivity (e.g., Japanese Patent Application No. JP2008-130729).
However, despite the radiation performance being secured during use (power generation), a cell case is subject to be heated in an unused state (non-power generation state) in the daytime by receiving heat at high outside air temperature. The cell temperature consequently rises and thermal degradation occurs. Specifically, when the cell temperature in unused state is higher than the outside air temperature, it is difficult to suppress to receive heat from outside so that there has been a problem that the cell capacity decreases despite non-use of the cell.